Related Rule
United States of America
Practice Relating to Rule 29. Medical Transports
The US Field Manual (1956) restates Article 35 of the 1949 Geneva Convention I and Article 21 of the 1949 Geneva Convention IV. 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, §§ 236 and 260.
The US Air Force Commander’s Handbook (1980) provides that ambulances … “should not be deliberately attacked, fired upon, or unnecessarily prevented from performing their medical duties”. 
United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 3-2.
The Handbook further stresses that medical transports lose their special immunity if they are used to commit “acts harmful to the enemy outside their humanitarian functions”. In this respect, the manual gives the example of “firing at the enemy from an ambulance”. 
United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 3-2(d).
The US Naval Handbook (1995) states: “medical vehicles … may not be deliberately bombarded. Belligerents are required to ensure that such medical facilities are, as far as possible, situated in such manner that attacks against military targets in the vicinity do not imperil their safety.” 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 8.5.1.4.
The manual qualifies “deliberate attack upon … medical vehicles” as a war crime. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 6.2.5.
The US Naval Handbook (2007) states that “medical vehicles … may not be deliberately bombarded. Belligerents are required to ensure that such medical facilities are, as far as possible, situated in such a manner that attacks against military targets in the vicinity do not imperil their safety.” 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, § 8.9.1.4.
The Handbook also states that examples of war crimes that could be considered as grave breaches of the 1949 Geneva Conventions include: “Deliberate attacks upon … medical vehicles.” 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, § 6.2.6(7).
Upon signature of the 1977 Additional Protocols I and II, the United States declared:
It is the understanding of the United States of America that the terms used in Part III of [the 1977 Additional Protocol II] which are the same as the terms defined in Article 8 [of the 1977 Additional Protocol I] shall so far as relevant be construed in the same sense as those definitions. 
United States, Declaration made upon signature of the 1977 Additional Protocols I and II, 12 December 1977, § B.
In 1987, in submitting the 1977 Additional Protocol II to the US Senate for advice and consent to ratification, the US President expressed the view that the obligations in the Protocol were “no more than a restatement of the rules of conduct with which US military forces would almost certainly comply as a matter of national policy, constitutional and legal protections, and common decency”. 
United States, Message from the US President transmitting the 1977 Additional Protocol II to the US Senate for advice and consent to ratification, Treaty Doc. 100-2, 29 January 1987, Comment on Article 10.
The US Field Manual (1956) restates Article 36 of the 1949 Geneva Convention I and adds:
It is not necessary that the aircraft should have been specially built and equipped for medical purposes. There is no objection to converting ordinary aircraft into medical aircraft or to using former medical aircraft for other purposes, provided the distinctive markings are removed. 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 237.
The US Air Force Pamphlet (1976) states:
Generally, a medical aircraft (identified as such) should not be attacked unless under the circumstances at the time it represents an immediate military threat and other methods of control are not available. For example, this might occur when it approaches enemy territory or a combat zone without permission and disregards instructions, or initiates an attack. Attacks might also occur when the aircraft is not identified as a medical aircraft because of lack of agreement as to the height, time and route. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 4-2(f).
The Pamphlet further provides: “In addition to grave breaches of the Geneva Conventions of 1949, the following acts are representative of situations involving individual criminal responsibility: (1) deliberate attack on protected medical aircraft”. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 15-3(c)(1).
The US Air Force Commander’s Handbook (1980) provides:
Medical aircraft, recognized as such, should not be deliberately attacked or fired on. Medical aircraft are not permitted to fly over territory controlled by the enemy, without the enemy’s prior agreement. Medical aircraft must comply with requests to land for inspection. Medical aircraft complying with such a request must be allowed to continue their flight, with all personnel on board, if inspection does not reveal that the aircraft has engaged in acts harmful to the enemy or otherwise violated the Geneva Conventions of 1949. 
United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 3-2(c).
The US Naval Handbook (1995) qualifies “deliberate attack upon … medical aircraft” as a war crime. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 6.2.5.
The US Naval Handbook (2007) states that examples of war crimes that could be considered as grave breaches of the 1949 Geneva Conventions include: “Deliberate attacks upon … medical aircraft.”  
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, § 6.2.6(7).
It is reported that in the Vietnam War, US army medical evacuation helicopters marked with the red cross emblem suffered a high loss rate from enemy fire, with the result that some medical evacuation units armed their helicopters with machine guns. 
Peter Dorland and James Nanney, Dust Off: Army Aeromedical Evacuation in Vietnam, Center of Military History, United States Army, Washington D.C., 1982, p. 85.
In 1987, the Deputy Legal Adviser of the US Department of State affirmed: “We support the principle that known medical aircraft be respected and protected when performing their humanitarian functions.” He added: “That is a rather general statement of what is reflected in many, but not all, aspects of the detailed rules in Articles 24 through 31, which include some of the more useful innovations in the Protocol.” 
United States, Remarks of Michael J. Matheson, Deputy Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, p. 423–424.
The Report on US Practice notes that US practice suggests that if enemy forces do not respect the protected status of medical units, the right of self-defence may justify the use of force. 
Report on US Practice, 1997, Chapter 2.7.
The US Air Force Pamphlet (1976) states: “In addition to grave breaches of the Geneva Conventions of 1949, the following acts are representative of situations involving individual criminal responsibility: (1) deliberate attack on … hospital ships”. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 15-3(c)(1).
The US Air Force Commander’s Handbook (1980) provides that ambulances and hospital ships “should not be deliberately attacked, fired upon, or unnecessarily prevented from performing their medical duties”. 
United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 3-2.
The US Naval Handbook (1995) qualifies “deliberate attack upon hospital ships” as a war crime. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 6.2.5.
The US Naval Handbook (2007) states that examples of war crimes that could be considered as grave breaches of the 1949 Geneva Conventions include: “Deliberate attacks upon … hospital ships.” 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, § 6.2.6.
The US Manual on Detainee Operations (2008) states:
Legal Considerations
a. As a subset of military operations, detainee operations must comply with the law of war during all armed conflicts, however such conflicts are characterized, and in all other military operations …
c. The four Geneva Conventions of 1949 are fully applicable as a matter of international law to all military operations that qualify as international armed conflicts … The principles reflected in these treaties are considered customary international law, binding on all nations during international armed conflict. Although often referred to collectively as the “Geneva Conventions,” the specific treaties are:
(2) [1949] Geneva Convention [II] … This convention … protects hospital ships. 
United States, Manual on Detainee Operations, Joint Chiefs of Staff, 30 May 2008, pp. I-2–I-3.